Several processes are already known aiming at bringing a vehicle onto a reference or nominal trajectory, or in other words to bring about coincidence between a trajectory actually followed by the vehicle and a theoretical trajectory.
In general terms, the theoretical trajectory is modeled by a succession of rectilinear or curved segments with a constant or non-constant radius of curvature, which join at position finding or localization points identified by beacons. French Patent 89 08049 describes a certain number of means which can serve as beacons and in particular radiophonic wave transmitters, reflecting posts or magnets buried in the ground. No matter what the localization method adopted, absolute position fixings for the vehicle, also referred to as localizations and which make it possible to deduce the variation between the real vehicle position and the localization point, are effected by appropriate sensors of the vehicle when the latter passes close to these localization points.
Once a deviation has been determined, a known relocking process consists of calculating a joining section to the theoretical trajectory leading to one of the following localization points thereof. For this purpose the coordinates of a certain number of intermediate points between which the joining section is broken down are calculated. If it is wished to avoid excessively sudden variations of the course of the vehicle and bring about a relatively rapid convergence to the theoretical trajectory, the number of intermediate points and therefore the calculation volume must be considerably increased, which may make it necessary to temporarily stop the vehicle.
Once the localization has been made and the variation with the localization point calculated, another process consists of making a vehicle follow a connection section to said localization point and not one of the following points. In certain cases, this method involves transverse displacements of the vehicle, which are sometimes impossible or only achievable with great difficulty.